gyrator930

2001 4.0, for lack of a better term, it runs poorly. NVH levels are very high, particularly on cold start, it will rattle every interior panel. Runs somewhat smoother in the 1500-2500 rpm range and then gets worse above 3000. Has a full tuneup including new coil rail, spark plugs, o2 sensors. Injectors were replaced since the problem started with no effect. Compression checks out perfect. At one point it did throw misfire codes for cylinder 2 and 5 when holding the engine at 3000 rpm, I couldn't determine the root cause and neither could 2 shops, code went away after about 2 days. Other than that, no codes. Harmonic balancer, flex plate and CPS are new. Oil pressure is perfect.

Any ideas/diagnosis tips on what else could be causing this before I just rebuild the whole thing? NVH issues can be a real pain in the ***

Tbone289

How did the old spark plugs look when you pulled them? You should compare used spark plugs to spark plug charts (google this) any time you pull them to check fuel mixture/ignition conditions in each cylinder.

As the engine is running, pull the fuel injector connectors off the injectors, 1 at a time, and see if the running condition (roughness) changes with each one. Pull connector off #1, check, replace connector, pull connector off #2, replace...repeat. If you run across a cylinder where this doesn't really affect how smooth the engine runs, you likely have a fueling issue in that cylinder.

How many miles do you have on your XJ?

Is performance definitely down, or are you just having vibrations? How is your MPG?

gyrator930

I'm no expert at reading spark plugs but comparing to photos of various spark plug charts they looked perfectly normal to me.

190k on the jeep, I'd say performance is reduced from what it used to be, but that's just a butt dyno guess, no data to really back that up. Gas mileage is fairly normal, 14-15 range which it has been for pretty much as long as I've owned the jeep.

I'll do the injector test for sure

Tbone289

If it's truly just an NVH issue, and not a performance issue, it could just be motor mounts or transmission mount. Check the appearance of those for cracked rubber, loose or broken bolts, etc.

gyrator930

All mounts are new. The symptoms do seem to be purely the perceived harshness of the engine, although it does start hard occasionally

Tbone289

That would still lead me toward a fuel issue at this point. Let us know how the injector testing goes.